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    <title>Document</title>
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<body>
    <p id="p"></p>
    <script>
        /* 2019-09-19 09:00:00 */
        setInterval(function(){
            var oDate=new Date();
            var playTime=new Date("2019-09-19 09:00:00").getTime();//毫秒
            var mill=oDate.getTime();
            var gapTime=playTime-mill;
            var seconds=Math.floor(gapTime/1000);
            var hours=Math.floor(seconds/3600);
            seconds=seconds%3600;
            var minutes=Math.floor(seconds/60)
            seconds=seconds%60;
            function transform(params){
                return params<10?"0"+params:params;
            }
            var sum=transform(hours)+":"+transform(minutes)+":"+transform(seconds);
            document.getElementById("p").innerHTML=sum;
           
        },1000)
    </script>
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